An earphone, or earpiece, is often used with portable digital products such as MP3 players, mobile phones, personal digital assistants (PDA) and laptop computers. An earpiece generates sound through magnetic fields generated around the earpiece. A schematic of magnetic fields generated by a conventional earpiece is illustrated by FIG. 1.
Referring to FIG. 1, the earpiece (not numbered) includes a magnet 110, which is comprised of two parallel magnets 102 and 104. The north and south poles of the magnets 102 and 104 set up a first magnetic field on an axis A. Since the magnets 102 and 104 are permanent magnets, the first magnetic field is constant and uniform. A magnetic force associated with the first magnetic field is perpendicular to the axis A. The earpiece also includes a balanced armature 130, which comprises a stationary portion 130a, an extending portion 130b and an arc portion 130c. The stationary portion 130a is connected to the frame (not shown) of the earpiece. The extending portion 130b is capable of moving about the arc portion 130c. When the extending portion 130b is centered in the first magnetic field, no net force is exerted on it.
The earpiece further comprises a coil 120. When a current passes through the coil 120, a second magnetic field is created on an axis B. The direction of the second magnetic field is perpendicular to that of the first magnetic field, i.e., axis B is perpendicular axis A. A magnetic force associated with the second magnetic field is perpendicular to the axis B. Accordingly, a net magnetic force may be generated to cause the extending portion 130b to rotate about the arc portion 130c. The extending portion 130b then transmits movement to a diaphragm 140 via a connecting rod 150. Vibration of the diaphragm 140 generates sound in the earpiece. Because the net magnetic force is in a different direction than the moving direction of the extending portion 130b, the efficiency of the magnetic energy is reduced.